SUBST
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syntax: (subst <new> <old> <tree>
[:test <test>]) ;Common Lisp
(tree-substitute <new> <old> <tree>
[:test <test>]) ;Babylon Lisp
This function exchanges the old elements of the list
<tree> against new ones. It does not only work on the
list <tree>, but it works too on all lists, which are
included in <tree>.
example: >(subst 12 1 '((21 1) 3 1))
((21 12) 3 12)